Project Summary Acquired or initial resistance is a major obstacle in the treatment of patients with cancer. One mechanism of treatment resistance in tumors is the change of DNA repair pathways, especially the pathways that repair double strand breaks (DSBs). We have gathered data implicating the transcription termination factor XRN2 in the response to and repair of DSBs. XRN2 is a 5'-3' RNA endonuclease that degrades RNA and resolves R- loops during transcription. We observed that loss of XRN2 leads to increased DSB formation, replication stress and R-loop formation. We also found that cells lacking XRN2 displayed increased sensitivity to PARP1 inhibition and exposure to ionizing radiation (IR). Loss of XRN2 led to a defect in the non-homologous end- joining pathway of DSB repair, which may be a contributing factor to IR sensitivity. However, we do not know the mechanistic role of XRN2 in NHEJ DSB repair, which will be studied in this project. Although the mechanism by which XRN2 promotes cell death upon PARP1 inhibition is unresolved, preliminary evidence suggests a role for XRN2 in the homologous recombination pathway of DNA repair, which will also be investigated in this project. Since IR exposure and PARP1 inhibitors are both used clinically as treatments for patients with tumors, especially glioblastoma multiforme, we will also determine whether loss of XRN2 can be part of respective combination therapies to effectively curtail tumor growth in vivo. The overarching goal of this proposal is to study how DNA repair pathways can be manipulated to enhance cancer radio- and chemotherapies. Results from our investigation will provide novel insights into the roles of XRN2 in DNA repair pathways with potential translational impact.